The present invention relates to single-channel search radars, in general, and more particularly, to apparatus for use therein which improves the estimation of the angle of a detected target within the search scan of the radar.
Most modern aircraft having weapon delivery systems generally employ a search radar for detecting targets of interest. In searching for a target, these radars usually scan through a spatial area with a plurality of looks or beam search samples. At each look, the radar may derive the amplitude of signals reflected from the target within the spatial area. Thereafter, the search radar may compute an estimated target scan angle from the amplitudes derived through the search scan. In turn, this estimated angle may be used to provide direction to the weapon delivery system for deployment of projectiles toward the target, for example.
An illustration of a typical scenario with regard to detecting a target is depicted in FIG. 1. Suppose that the aircraft, denoted at 10, is flying along a flight path 12 in the direction indicated by the solid arrow 14. At a flight position P1, the search radar on-board the aircraft 10 may scan a spatial area 16 with its radar beam 18 in search of a target depicted in the figure as the dot 20. In its search, the beam 18 of the radar may be scanned through a plurality of looks L1, L2, . . . , L9 corresponding to a plurality of scan angles .theta.1, .theta.2, . . . , .theta.9. At each look L1, the search radar may correspondingly derive an amplitude ai of the radar signal reflected from the target 20. An idealistic example of a plot of amplitudes a.sub.i for the present example may appear as that shown by the x's on the dashed line in the graph of FIG. 2.
Referring to FIG. 2, in some search radars, a simple centroiding procedure having the formula .SIGMA.a.sub.i a.sub.i /.SIGMA.a.sub.i, for example, has been used to compute the estimated target angle .theta..sub.t which, of course, falls between the scan angles .theta.5 and .theta.6 for the aircraft position P1 in the present example. Accordingly, as the aircraft 10 moves to another position P2, another scan of looks may be performed and corresponding amplitudes computed by the search radar. Similarly, a curve of amplitudes for the search scan at position P2 may be compiled as that shown by the second dashed line (P2) curve in FIG. 2. It follows that the computed centroid of this second curve (P2) will be the estimated target angle with respect to the new aircraft position P2.
While for an ideal case, this simple centroiding procedure appears adequate for accurately estimating the true target angle for weapon delivery, it is evident that in more practical cases, the accuracy of the target angle estimation with this method may be somewhat degraded. For example, under most conditions, the aircraft search radar incurs undesirable noise at the input stages of the search radar itself. It happens that this instrumentation noise is inseparable from the echo signals returned from the target and thus tends to effect relatively large errors in the computation of the amplitude measurements of the target reflections through the various search beam directions. To further complicate matters, there is no guarantee that the beam scanning samples or looks will be scanned symmetrically about the true target angle. Moreover, even greater inaccuracies with the centroidal method can be expected when target scintillation provides further adverse noise sources.
Apparently, in view of the practical problems of noise as discussed above, the actual amplitude measurements derived by the search radar are not expected to follow any ideal curve fitting pattern for most practical sets of conditions. For example, the graph of FIG. 3 illustrates a case in which actual amplitude measurements r(.theta..sub.i), denoted by X's, do not coincide with the ideal 1 amplitude measurements s(.theta..sub.i) denoted by the dots lying substantially on the dashed line curve. In this case, it is quite apparent that the simple centroid of the actual amplitude measurements will not result in an accurate estimation of the true target scan angle. Consequently, if the calculated simple centroid was used as the true target angle, it would cause an erroneous deployment angle for the weapon delivery system of the aircraft, for example.
From the above, it is evident that to be a viable piece of equipment for enhancing the effectiveness of weapon deployment, as one example, the search radar of the aircraft must accurately estimate the true scan angle of the target under even the most adverse conditions of noise with regard to both the aircraft and target flights and the internal operations of the radar itself. To accomplish this, it is felt that more sophisticated apparatus beyond that of a simple centroiding method is needed to process the actual amplitude measurements as derived by the search radar.